The invention relates to a method and system for drying material, in which the material is dried by means of negative pressure and a high-frequency electromagnetic field generated by electrodes in a space isolated from its surroundings.
Drying techniques exist that try to shorten the drying times of materials using negative pressure and micro-wave heating. These drying systems use different types of fixed autoclaves and different systems for discharging the material being dried into the external pressure. In these prior-art solutions, the electrodes generating the high-frequency field are normally attached to the autoclave. The fixed structure of the known systems limits the modifiability of the drying systems for pieces of different sizes and shapes and also constitutes a heavy assembly which is not easy to move.
Published U.S. Pat. No. 4466198 describes a drying system in which the material to be dried is, instead of a rigid-structure autoclave, enclosed in a flexible covering to which a first electrode is attached while a second electrode resides outside the covering. Moisture from the lumber being dried is led out of the covering through the first electrode. The described system is a lighter and easier-to-move application than the previous drying systems, but its drawback is still a fixed drying space; the covering is at its lower part attached to the lower electrode and it cannot be moved independently in relation to the electrodes. The material to be dried is thus placed into the drying space and removed from it after the drying ends, so the system does not enable a continuous flow of material being dried through the electrodes, or the storage of the material in a closed drying space after the drying.
It is an object of the invention to develop a drying method and drying system so as to solve the above-mentioned problems. The object of the invention is achieved by a method and system which are characterized by what is stated in the independent claims. Preferred embodiments of the invention are set forth in the dependent claims.
The invention is based on using as drying space a drying batch-specific film package substantially separate from the rest of the equipment. During drying, a negative pressure is provided in the film package and the material in the package is subjected to an electromagnetic field generated by external electrodes. For the period of drying, the film package is connected through movable hoses to equipment for removing moisture from the package. After drying, the film package is removed from the drying system and it can be used to protect the material in storage or transport.
The system of the invention provides the advantage that it is simple and light and the drying batch-specific film packages can be moved, which makes it possible to convey material batches packed in film packages continuously or in stages through the effective area of the electrodes. Because the drying space formed by the film package is drying batch-specific and separate from the rest of the equipment, material can after drying also be stored in the film package for transport, storage or evening-out of moisture. Products dried and packaged in manufacturing by means of the method of the invention keep without the damaging effect of micro-organisms until they are taken into use. The system of the invention also provides the advantage that it is suited for field use, because the separate parts of the system are easy to move and to construct onto a conveying means. The drying system of the invention is also easily modifiable for material pieces of different sizes and shapes, since contrary to the prior art, the dimensions of the piece to be dried are not limited by the drying space. An additional advantage of the system of, the invention is that measurement is easy, because the material to be dried is not surrounded by fixed and heavy structures hampering measurement.